A conventional multi-battery battery charger typically charges the batteries inserted therein using the same charge profile. To illustrate this, an example in which two batteries are charged is discussed next in connection with FIGS. 1, 2 and 3, which respectively show individual and aggregate charge profiles for a prior art technique in which Batteries A and B, each with a charge capacity of twenty-four hundred (2400) milliamp hours (mAh), are concurrently charged for thirty (30) minutes. In FIGS. 1-3, the x-axis represents the charge time in units of minutes, the y-axis represents the charging current in units of Amperes (A), and the curves represent the charge profiles, or the charging current as a function of the charge time.
From these figures, initially eight (8) A of current is supplied to charge Battery A (FIG. 1) and eight (8) A of current is supplied to charge Battery B (FIG. 2) during a first charging stage. The charging current for both of the Batteries A and B is then decreased together to four (4) A at the ten (10) minute mark for a second charging stage and to 2 (2) A at the twenty (20) minute mark for a third charging stage. The charging current at the beginning of charging typically is higher and decreases as the batteries charge due to the increases in battery temperature as the battery charges. Unfortunately, this technique requires the battery charger to draw at least sixteen (16) A of current (FIG. 3) when charging begins to fully charge both batteries (forty-eight hundred 4600 mAh in thirty (30) minutes), and the cost of the charging circuitry generally follows the maximum current draw of the charging circuitry.
In addition to the above, battery temperature is also controlled by cycling or pulsing the charging current, for example, using a duty cycle of fifty percent (50%) wherein for each pulse the charging current is supplied 50% of the time and not supplied the other 50% of the time. Unfortunately, this results in periods of time when neither of the Batteries A and B are being charged, leading to charging inefficiencies.